1. Field of the Invention
The present invention generally relates to a circuit for a differential amplifier. More particularly, the present invention relates to a system and method for controlling output-common-mode voltages in differential amplifiers without tail currents.
2. Background Art
An important advantage in transitioning to finer-geometry complementary metal oxide (CMOS) processes is the ability to use low-voltage power supplies, which corresponds to a lower-power operating environment. Although CMOS processes fulfill the need for low-power operating requirements and consumption, a need still exists to be able to achieve large signal swings from these CMOS amplifiers in order to maintain adequate signal-to-noise ratios. Several conventional approaches to providing differential amplifiers with reasonably large output swings exist. A few of these traditional approaches are discussed in greater detail below.
Several metrics exist for rating and measuring the overall performance of differential amplifiers. Among these metrics are total current, slew current, and output swing. As known in the art, differential amplifiers include an inverting input and a non-inverting input and corresponding inverting and non-inverting outputs. The total current for differential amplifiers, therefore, is measured by adding the absolute value of the current detected at both the inverting and non-inverting outputs.
Slew current is defined as the amount of current detected at one amplifier output port when completely tilting one input to the other. Typically, slew current is used as a measure of an amplifier's efficiency. Another important metric is the output swing, which is a measure of how far the amplifier can go from rail to rail before it begins losing gain. Although a number of different techniques exist for providing large signal output swings in CMOS amplifiers, each of these techniques suffers from at least one major disadvantage.
For example, while certain amplifiers may be able to produce large output swings, these same amplifiers may be slow or inefficient. Other amplifiers that may be faster, may sacrifice efficiency for speed. Finally, other amplifiers that may provide reasonably large output swings and high efficiency, may be unable to reject output common-mode voltage disturbances. Common-mode voltage, as understood in the art, may be created as a result of imbalances in transistor performance and power supply variations. Common-mode voltage may also result from the input source directly or from a previous amplification stage.
Although particular types of differential amplifiers provide tail current sources as an effective technique for resolving problems associated with common-mode voltages, tail current sources create inefficiencies and inherently reduce headroom swing in these amplifiers. Therefore, a need exists to be able to derive high output swings from low-power power supplies in CMOS differential amplifiers without tail current sources.